The Physics of Catching the Fish

Home the rod, the reel, the line.



The halibut itself can exert multiple forces on the line including tension and shear. The force of tension that is put on the line by the fish is felt through the torque on the rod and partly absorbed by the frictional force in the reel. The fish also experiences the force due to the changes in pressure from being brought up from the bottom at a depth of up to 100 meters. For every 10 meters the fish is brought up from the bottom the fish experiences a change in pressure of 1 atmosphere. Fish that live at extreme depths have ways to deal with the problem of intense pressure but are not usually able to deal with extreme changes in pressure that occurs when they are brought up quickly. This is especially true of fish that use a swim bladder to deal with the problem of buoyancy. The problem of buoyancy is explained by Archimedes principle F_B=W_fluid. The buoyant force is the upward force that a fluid applies on an object that is partially or completely immersed in it. Since the halibut is completely immersed in seawater the buoyant force acting on the halibut is equal to the magnitude of the weight of the seawater that the halibut displaces. An average size halibut can weigh about 75 kilograms and be over a meter long. To see a chart on how to estimate the weight of a halibut of a known length click on the halibut below.
Halibut may use other methods of achieving neutral buoyancy at extreme pressures such as reduced bone weight. I truthfully don't know how halibut have overcome this problem but I do know they migrate long distances and usually live at extreme depths but migrate to as shallow as 20 meters during the summer.

The Physics of Fishing Line

Home the rod, the reel, the fish.
The fishing line used for halibut fishing must be considered for its ability to resist shear and tensile forces that are put on it by the fish. For the reason that halibut can put extreme tensile forces on the line and can shear the line with their sharp teeth, halibut fishing requires innovative technology to increase the fisherman's ability to land fish and keep from losing gear. Most sport fishing requires fishing line that is not very strong such as the mono filament lines used for freshwater and smaller saltwater sport fish that most people are used to. For halibut fishing special braided lines have been designed to hold up to extreme tensile forces while maintaining a small diameter so that a large amount of line can be placed on the reel. The equations that explain shear and tensile forces on the line are: F=S(∆x/L₀) and F=Y(∆L/L₀) in these equations the Y represents Young's modulus for the line that is being used on the reel and represents the force needed to break the line by changing its length. The S in the equation regarding shear forces describes the change in the shape of the line due to the teeth of the halibut. This is why a heavier piece of line is usually used at the end where the hook is attached. This makes it even more difficult for the fish to change the shape of the line enough to cause it to break. The line is weakest at the point where the knot attaches the line to the hook this is the most likely spot for the line to break when fighting the fish. The tension on the knot can be reduced by using a swivel to attach the hook to the line. The swivel allows the line to rotate and absorbs some of the tension that the fish is putting on the knot attaching the line. To see an example of how tensile line strength and knot strength are different click on the picture of the line below. Photo Credit: http://www.pennreels.com/01_products/line/pro.htm

The Physics of a Fishing Reel

Home the rod, the line, the fish.
The reel is able to produce a fair amount of torque on the fish as well. The reel’s main function however is to produce a heavy frictional force on the spool of line so that when the fish changes direction and starts to take line back off the spool the fish must first overcome the static frictional force and then continue to battle the kinetic frictional force the reel is putting on the spool of line as it swims away. By continually changing the fish’s direction with the torque exerted by the rod the fish starts to become tired and will allow itself to be pulled in the direction of the torque placed on it by the reel. Once a halibut realizes that the surface is close it is likely to change direction if it is not too tired already and make a run for the bottom again. This is where we will start to consider how this torque is being applied to the fish in the first place. The line is being brought in by winding it around a spool through a line feeder that moves back and forth as the spool spins to ensure that the line is distributed evenly on the spool. The line feeder is designed to move the distance of the average diameter of the line in the time it takes the spool to complete one period. This way the line does not pile up on one end and the most line possible can be used on the spool. The fisherman is creating the angular momentum of the spool by turning the handle of the fishing reel. This handle can be thought of as a crank that is turning gears inside the reel that in turn transfer the rotational energy to the spool. Check out this high end Penn saltwater reel they named the Torque series.


















The equations that explains the static frictional force that must be overcome by the fish to start to take line off the spool is f_s^max=μ_sF_N and the equation that explains the kinetic frictional force the reel exerts on the line as the fish swims away is fk=μkFN. The constants for kinetic and static friction which are denoted μk and μs are determined by the drag which is a component on the reel which is set by the fisherman. The drag should be set according to how much to how much force the fisherman wants to able to exert on the fish through the line before the spool will spin. Remember that you are also exerting this force on the tension in your line and on the knot where your line is attached to your hook.

The Physics of a Fishing Rod

Home the line, the reel, the fish.

The rod has the ability to produce a torque on the fish that can change the fish’s direction and make it possible to start reeling in the line. There is a frictional force exerted on the line by the guides on the rod.

Photo Credit: http://www.pennreels.com/01_products/rods/intv_ara_standuprods/intv_ara_standuprods.htm

On most halibut rods the guides have rollers that greatly reduce this frictional force. This is almost always the case for the top guide on a halibut rod. Halibut rods are generally short and stiff to provide the greatest amount of torque possible when fighting the fish. Since the line is fished right underneath the boat there is no need to cast the line out, making a shorter rod more favorable for this reason as well. The fisherman’s wrist acts as a point of rotation to make it possible to exert a torque on the fish when the time comes to “set the hook”. The equation that explains the torque exerted on the line by the rod is τ=Fl. Where l is the distance between the line at the tip of the rod and the axis of rotation (the fisherman's wrist) multiplied by the sin of the angle between the rotation point and the tip of the rod. The force is exerted by the fish on the end of the line. At a point when the fish is about to change direction the system is momentarily in equilibrium and the sum of the torques on the system is equal to zero. Στ=0! This means that the torque being placed on the fish by the tension in the line is exactly equal to the torque that the fish is exerting back on the line.

Physics of Catching Halibut Sport Fishing

Welcome to the Physics of Catching Halibut while Sport Fishing website. Here we will explore all the physics concepts that apply to a fisherman who is attempting to catch a halibut from a boat with sport fishing tackle. This topic will be broken down into the four key components that must be considered when exploring the variables associated with the physics of halibut fishing. These subtopics include the rod, the reel, the line, and the fish. The equations that apply to each of these components and any physics terminology that applies will be explained for each of these subtopics on subsequent pages. For now I will say that sport fishing for halibut is a popular recreational pastime, especially in Southeast Alaska where these photos are from: The physics principles associated with this awesome sport are very rarely considered by unexperienced fisherman and even experienced fisherman might not consider how this process relates to the concepts of physics at the level that will be considered here. To find out more about halibut fishing around the Juneau area go to the Department of Fish and Game's Website. For information about halibut fishing in other places in the Pacific Northwest check out halibut.net for season dates and other information. Picture credit: http://www.clas.pdx.edu/newsletters/0906/0906_alaska_fishing.jpg